Wait... how can you get a "most lethal weapon"? Are we talking percentage death rate, speed of death, severity of death, what? And you can't say "ALL OF THEM" because what works for one doesn't work for another.

Chlorine gas is great at severe injury, in widespread areas. But it takes time to do it's job, and also it isn't certain that it'll cause death. A sledgehammer will cause death, but only to one person. An uzi will cause widespread death, but it won't be too severe or painful.

just kidding.
its mankind, because without mankind, no weapons would exist, including the human mind. creepy.

actually...

did you know that we didnt create electricity? it created US! because scientists believe that the first single-celled organisms were created from lightning strikes. we evolved from those organisms, so the greatest weapon is electricity.

but lightning comes from clouds, which come from moisture, which come from water.